Effect of Cuprous Oxide Nanocubes and Antimony Nanorods on the Performance of Silicon Nanowire-Based Quasi-Solid-State Solar Cell

Antimony nanorods (SbNRs) anchored to vertically aligned SiNWs serve as cosensitizers and enhance the light absorption of NWs, and their favorably positioned valence band (VB) coupled with their p-type semiconducting nature allows fast hole extraction from SiNWs. Photocorrosion of SiNWs is effectively prevented by a monolayer of N-[3-(trimethoxysilyl)propyl]aniline (TMSPA). Upon assembling a quasi-solid-state solar cell with a SbNRs@TMSPA@SiNW photoanode, a triiodide–iodide (I3–/I–) redox couple-based gel encompassing dispersed p-type cuprous oxide nanocubes (Cu2O NCs) as the hole transport material. and an electrocatalytic NiO as the counter electrode, a power conversion efficiency (PCE) of 4.7% (under 1 sun) is achieved, which is greater by 177% relative to an analogous cell devoid of the Cu2O NCs and SbNRs. SbNRs at the photoanode maximize charge separation and suppress electron–hole and electron–I3– recombination at the photoanode/electrolyte interface, thereby improving the overall current collection efficiency. Concurrently, the Cu2O NCs facilitate hole scavenging from SbNRs or SiNWs and relay them rapidly to the I– ions in the electrolyte. Optically transparent and mesoporous NiO with a VB conducive to accepting electrons from FTO permits abundant interaction with I3– ions. The high PCE is a cumulative outcome of the synergistic attributes of SbNRs, Cu2O NCs, and NiO. The SbNRs@TMSPA@SiNWs/Cu2O-gel/NiO solar cell also exhibits a noteworthy operational stability, for it endures 500 h of continuous 1 sun illumination accompanied by an ∼24.4% drop in its PCE. The solar cell architecture in view of the judiciously chosen components with favorable energy level offsets, semiconducting/photoactive properties, and remarkable stability opens up pathways to adapt these materials to other solar cells as well.


■ INTRODUCTION
Silicon nanowires (SiNWs) are promising photoanode scaffolds in liquid junction solar cells (LJSCs), wherein the vertically aligned nanowires with high surface areas trap the maximum possible visible light for conversion, resulting in high power conversion efficiencies (PCEs) in the range of 6− 13%. 1−5 One of the most attractive features of SiNWs is their one-dimensional structure, which allows radial junction hole transport and transfer, and other pertinent properties include low reflectivity (less than 5% in the visible region), 6−8 synthetic ease via metal-catalyzed n-Si wafer etching, and the ability of the SiNWs to deliver high PCEs, despite surface defects and impurities, a property that planar-Si solar cells cannot boast of. 9,10 The simplest configuration of an LJSC comprises a photoanode of SiNWs grown directly over a n-Si wafer in direct contact with a hole transport layer (HTL), with the most widely used one being an aqueous solution of HBr and Br 2 , exposed to an electrocatalytic counter electrode (CE), usually Pt on the other side. 11 However, this cell, SiNWs/ HBr,Br 2 /Pt mesh, having a PCE of 0.29% 11 suffers from several drawbacks, the principal one being the extremely high corrosivity of the acid electrolyte, which chemically erodes the SiNWs leading to the rapid decline in the cell performance within a few hours. 2 Over the years, this architecture has been modified for both improved PCE and stability. These approaches include the anchoring of catalytic Pt or Au metal nanoparticles to SiNWs 1 by coating SiNWs with a layer of carbon to protect them from the electrolyte attack during cell operation and then decorating them with Pt NPs for improved charge separation ensuing in a record PCE of 10.86%. 3 Some interesting configurations that replaced the corrosive Br 2 /Br − HTL with the conducting polymer, poly (3,4ethylenedioxythiophene):poly(4-styrene sulfonate) (PE-DOT:PSS), and still retained reasonably high efficiencies of ∼6.9% with tapered SiNW, 12 9.3% with optimal annealing and length conditions, 13 and 8.4% with short SiNW lengths of ∼0.4 μm and good coverage of PEDOT:PSS, 14 have been reported. Yoon et al. reported a hierarchical Si-and PEDOT:PSS-based hybrid solar cell having a siloxane interlayer. The device showed a very high efficiency of 17.34%. 15 Zhang et al. reported the use of a composite of polyethylenimine and cesium carbonate as an interlayer between the nanostructured silicon and Al metal at the rear side of the device. Cs 2 CO 3 served as an electron injection layer and thermal evaporation of Al on top, which resulted in the formation of Al−O−Cs bonds, which lowered the work function of Al. The device showed a PCE of 13.7%. 16 Shen et al. used copper thiocyanate and PEDOT:PSS on SiNWs as a double hole transport layer. The tetramethylammonium hydroxide-treated SiNW absorberbased device showed a PCE of 12.24%. 17 Zhang et al. reported a hybrid Si solar cell having a methyl/allyl organic monolayer as a surface passivating agent. The device delivered a PCE of 10.2%. 18 Pudasaini et al. used ultrathin atomic layer deposition (ALD)-grown Al 2 O 3 as a surface passivating layer on Si nanopillars for a hybrid solar cell. The device gave a PCE of 10.56% efficiency. 19 Wang et al. showed that a thin SiO 2 coating on SiNWs can act as a surface passivating layer and the cell produced a PCE of 12.4%. 20 Yet, another heterojunction structure of ITO/V 2 O 5 /n-SiNWs/TiO 2 /Al, wherein V 2 O 5 and TiO 2 serve as the holeand the electron-selective layers, delivered a PCE of 12.7% due to the light trapping effect of the wires and the excellent carrier transport due to the oxides. 21 Another notable report involved the covalent bonding of diallyl disulfide to the SiNWs via UV irradiance, imparting good air stability and PCE (7.2%) to the cell. 22 Some of our earlier configurations comprised of decorating SiNWs with hole-transporting nanostructures such as Se NPs (7%), C@Te nanorods (11.5%), and graphene quantum dots or GQDs (13.2%). 4,5,23 However, these cells employed the HBr, Br 2 electrolyte, thus limiting their practical applications, with some improvement in stability achieved subsequently, by the use of I 3 − /I − electrolytes at the cost of cell efficiency. 24,25 In view of the above developments and lacunae, this report expounds the fabrication of a stable and an efficient solar cell, with photoactive and hole-conducting antimony nanorod (SbNR)-decorated [3-(trimethoxysilyl)propyl]aniline (TMSPA)-passivated SiNWs as the photoanode, an I 3 − ,I − gel enriched with Cu 2 O nanocrystals as the HTL, and with a NiO CE. The highlights of this novel solar cell are (1) the low cost, ease of availability, and low toxicity of the elements (Sb, Ni, and Cu) in the photo/electroactive materials involved, (2) the ingenious approach of dispersing Cu 2 O in the gel to improve hole transfer and transport via favorable energy level offsets, (3) the application of the highly electrocatalytic and stable NiO CE film for maximum charge separation and therefore efficiency, and (4) the high stability induced by the TMSPA self-assembled monolayer that passivates the surface suppressing the rigorous oxidation of SiNWs in the electrolyte. 26 Contrasting with earlier studies, where costly Pt or Au NPs are used in the photoanode, and the expensive and opaque Pt mesh is used as the CE, 3 that too in an O-ring-type cell, thus restricting the cell's practical application; here, a quasi-solidstate cell is fabricated with an optically transparent NiO/FTO CE, thus allowing direct solar light penetration from the front side, enabling its real-world utilization. This unique inexpensive, innovative, stable cell architecture of SbNRs@ TMSPA@SiNWs/Cu 2 O-I 3 − ,I − gel/NiO@FTO opens up future possibilities for easily doable interface engineering via chemical methods for SiNWs with potential for scale-up and commercial application as well. ■ EXPERIMENTAL SECTION Chemicals. n-Type silicon wafers (CZ, 1−10 Ω cm) were purchased from Siegert Wafer GmbH, Germany. Fluorinedoped tin oxide (FTO) with a sheet resistance of ∼25 Ω cm 2 was purchased from Pilkington. Antimony chloride (SbCl 3 ), zinc (Zn) dust, copper nitrate trihydrate (Cu(NO 3 ) 2 ·3H 2 O), silver nitrate (AgNO 3 ), nickel nitrate hexahydrate (Ni(NO 3 ) 2 · 6H 2 O), hexamethylenetetramine or hexamine (C 6 H 12 N 4 ), lithium iodide (LiI), iodine (I 2 ), poly(ethylene oxide) (PEO, MW: 600 000), N- [3-(trimethoxysilyl)propyl]aniline (TMSPA), 3-propyl-1-methyl imidazolium iodide (PMII), 1ethyl-3-methyl imidazolium thiocyanate (EMISCN), hydrofluoric acid (HF, 40%), ammonia (NH 3 , 32%), hydrogen peroxide (H 2 O 2 , 30%), toluene (C 7 H 8 ), formic acid (H 2 CO 2 , 95%), hydrochloric acid (HCl, 37%), sulfuric acid (H 2 SO 4 , 98%), ethanol, acetone, and isopropanol were purchased from Merck. Ultrapure water with a resistivity of ∼18.2 MΩ cm was obtained from the Millipore Direct-Q3 UV system.
Etching of Silicon Wafer. The silicon wafers were cut into 1.5 × 1 cm 2 pieces, which were cleaned sequentially in acetone and deionized water and added to the boiling Piranha solution (H 2 SO 4 /H 2 O 2 = 3:1 v/v) to remove unwanted organic residues. After the cleaning process, the wafers were rinsed in deionized water and dipped in 5% HF solution to remove the SiO 2 layer from the surface. The etching was performed in a plastic beaker containing an aqueous mixture of 5 mol/L of HF and 0.02 mol/L of AgNO 3 for 15 min. The silver-coated etched silicon wafers were dipped in a solution of NH 3 and H 2 O 2 having a 3:1 ratio to remove the silver coating. Blackcolored etched silicon wafers were cleaned in distilled water and dried at room temperature and were ready to use. 27 Synthesis of Sb Nanorods and Cu 2 O Nanocubes. Antimony (Sb) nanorods were synthesized using a solvothermal method by following a previous report. 28 SbCl 3 (521 mg) was dispersed in 40 mL of toluene by vigorous stirring at 5000 rpm for 30 min and transferred to a 50 mL Teflon-lined stainless steel autoclave. Zn powder (75 mg) was added to the solution, and the autoclave was placed in a vacuum oven at 180°C for 12 h. The autoclave was allowed to cool for a further 6 h, the black precipitate was filtered, and the residue was washed thoroughly using absolute alcohol, dilute HCl, and deionized water sequentially. Finally, the product was dried at 50°C under vacuum and collected.
Cuprous oxide nanocubes (NCs, p-type) were synthesized using a hydrothermal method: 29 formic acid (4 mL) was added to a 0.05 M ethanolic solution (60 mL) of copper nitrate and ultrasonicated for 10 min. The solution was transferred to a Teflon-lined stainless steel autoclave (100 mL), sealed, and kept in an oven at 150°C for 2 h, cooled thereafter. A browncolored product was collected, washed in ethanol and distilled water several times, and dried at 60°C for 5 h.
Fabrication of NiO Counter Electrode (CE). FTO (SnO 2 :F)-coated glass substrates were cleaned sequentially in detergent, acetone, IPA, and deionized water. Optically transparent NiO counter electrodes were fabricated by a potentiostatic method 30 in an aqueous solution of 0.05 M hexamine, 0.05 M Ni(NO 3 ) 2 , and 0.05 (M) KCl in a threeelectrode system having FTO-coated glass as the working electrode, Pt rod as the counter electrode, and Ag/AgCl/KCl as the reference electrode. A constant dc potential of −1.1 V was applied for 600 s, and a light green-colored Ni(OH) 2 compound was grown over FTO. Ni(OH) 2 -coated FTO electrodes were annealed at 350°C to obtain a brown-colored NiO electrode. Solar Cell Fabrication. The surface of the SiNWs was passivated by dipping etched SiNW wafers in a 0.05 M ethanolic solution of TMSPA (N- [3-(trimethoxysilyl)propyl] aniline) for 24 h, dried at 60°C, and TMSPA-coated etched wafers were obtained. SbNRs were dispersed in isopropanol by ultrasonication for 1 h. The drop volume and concentration of the SbNR dispersion in isopropanol are 100 μL and 1 mg/mL, respectively. The dispersion was drop-cast over SiNWs, dried at 60°C, and used as a photoanode. A homogeneous liquid electrolyte containing 0.01 M I 2 , 0.02 M LiI, PMII (3.25 mL), and EMISCN ionic liquid (1.75 mL) in PC (20 mL) was prepared. The gel electrolyte was prepared by adding 5% (W/ V) PEO to the electrolyte without iodine under vigorous stirring. The gel formation took place at 60°C after 15 min of addition of PEO to the electrolyte. The hot gel electrolyte was allowed to cool down, and then iodine was added and mixed to form a uniform iodine−iodide-based gel electrolyte. A composite electrolyte was prepared by dispersing Cu 2 O NCs in the iodide-based liquid electrolyte along with PEO. A thick parafilm separator with a cavity size of 10 mm × 5 mm was placed over the NiO@FTO CE, and the gel electrolyte was applied into the cavity. SbNRs@TMSPA@SiNW electrode was placed on it. Electrical contact from the photoanode was taken through a copper wire/Ag paste on the back side of the photoanode. The device fabrication is shown in detail in Scheme S1 using cartoons and photographs.
Instrumental Methods. X-ray diffraction patterns of the materials were recorded on a PANalytical X'PertPRO instrument with Cu Kα (λ = 1.5406 Å). The morphology of the materials was studied on a field emission scanning electron microscopy instrument (Zeiss Cross beam 350 FEG-SEM with LASER-FIB). The UV−visible absorption spectra of the nanomaterials were recorded in the diffuse reflectance mode and converted to absorption using the Kubelka−Munk function on a UV−vis spectrophotometer equipped with an integrating sphere. Fluorescence spectra were collected on a Horiba Fluoromax-4 spectrophotometer. The transmission electron microscopy (TEM) images of Cu 2 O NCs and SbNRs were recorded on a JEOL 2100 microscope working at an accelerating voltage of 200 kV. Linear sweep voltammetry, cyclic voltammetry, chronoamperometry, and electrochemical impedance spectra (EIS) of the cells were studied on an Autolab PGSTAT 302N connected to a frequency response analyzer (FRA) and NOVA 1.11 software under an ac amplitude of 20 mV over the frequency range of 1 MHz to 0.01 Hz. I−V measurements of solar cells were recorded on a Newport Oriel 3A solar simulator with a Keithley model 2420 source meter. A 450 W xenon arc lamp was used as a light source with a light intensity of 100 mW cm −2 and of Air Mass (AM) 1.5 G illumination; the spatial uniformity of irradiance was determined by calibrating with a 2 cm × 2 cm Si reference cell traceable to NREL and reaffirmed with a Newport power meter. EQE versus wavelength spectra were recorded for the LJSCs over a wavelength range of 360−1040 nm on a quantum efficiency measurement system, an Oriel IQE-200 compliant with ASTM E1021-06. A 250 W quartz tungsten halogen lamp functions as the light source, the monochromator path length was 1/8 m, and the spot size was 1 mm × 2.5 mm rectangular at focus. ■ RESULTS AND DISCUSSION Structures of SbNRs and SiNWs. Prior to analyzing the structure, the energetics of the full solar cell, SiNWs/TMSPA/ SbNRs/Cu 2 O-I 3 − -I − /NiO/FTO, is discussed and the energy band diagram is shown in Scheme 1. At the photoanode, the photogenerated electrons from both SiNWs and SbNRs reach the CE, where I 2 or I 3 − species reduce to I − at the NiO/ electrolyte interface. I − ions then diffuse to the photoanode, get oxidized, and regenerate the SiNWs and SbNRs. Concurrently, Cu 2 O NCs, which are dispersed in the electrolyte, facilitate hole transfer and transport, owing to their p-type conducting nature and their favorably positioned Fermi level (E F ) at −4.5 eV. 31 Hole transfer occurs in the following direction: from the VB of SiNWs to the VB of SbNRs and then to I − via the E F of Cu 2 O or directly to I − ensuring circuit closure. All valence band/conduction band (VB/CB) positions were determined from CV plots ( Figure S1 and Table S1) and optical band gaps.
The XRD pattern of Sb nanorods ( Figure 1a Pure SiNWs (Figure 1b) show an extremely strong and intense reflection at 2θ = 69.5°, which is assigned to the (400) Scheme 1. Energy Band Diagram of the SbNRs@TMSPA@ SiNWs/Cu 2 O-Gel/NiO Cell Architecture plane of highly crystalline and well-oriented SiNWs with a facecentered cubic (fcc) lattice structure as per JCPDS: 892955. The inset displays the pattern obtained for Sb nanorods deposited over SiNWs with two sharp peaks at 2θ = 33.21 and 61.87°, which arise from the (012) and (311) planes of the rhombohedral phase of Sb and a third peak at 69.5°that originates from the (400) plane of SiNWs. The (400) plane has a broad base with a shoulder at 71°stemming from the  The oxygen functionalities and Sb interact via van der Waals attractive forces, which possibly causes these peak shifts for Sb. The diffuse reflectance spectra in Figure S2 show that the average reflectances of planar Si and SiNWs are ∼55 and ∼5%, respectively. This lowering in the reflectance of SiNWs is due to the light trapping property of vertically grown nanowires.
FE-SEM images of SiNWs ( Figure S3, Supporting Information) reveal vertically aligned nanowires, which are slightly tilted in some regions; the wires are densely packed and have diameters in the range of 100−200 nm and 3−7 μm in lengths. The EDAX plot ( Figure 1c) shows distinct signals due to Si, C, and O, and their elemental proportions are 73.8, 19.8, and 6.4%. The surface of SiNWs is oxidized to some extent upon exposure to an ambient atmosphere, which is rich in dissolved oxygen and moisture, and this contributes to the oxygen signal. Cross-sectional FE-SEM image of SbNRs ( Figure 1d) exhibits clusters of juxtaposed elongated structures, with lengths and diameters in the ranges of 1−3 μm and 100−350 nm, respectively. The rods have a rough texture, appear to be hollow and open-ended, and the average thickness of the walls is approximately 15 nm. Similar hollow Sb nanotubes have been reported by Li et al. 28 The solvent toluene functions as the structure directing agent and first induces the formation of lamellar layers of covalently bonded Sb atoms, which then roll up under solvothermal conditions to form the rods. The Zn 2+ ions and solvent molecules are adsorbed on the layers, and they cumulatively provide the driving force for the formation of rodlike shapes. To minimize the surface energy and the thermal stresses generated during the heat treatment, the Sb layers curl up and form these shapes. 28 The high-resolution TEM images of SbNRs ( Figure  1e) show the rodlike structures to be composed of Sb nanoparticles, 10−40 nm in dimensions, almost fused to each other with indistinct grain boundaries and their hollow structure is evident in Figure 1f. Figure  Absorbance, Fluorescence, and Energetics of Photoanode Components. The UV−visible absorption spectra of SbNRs, SiNWs, and SbNRs@SiNWs are displayed in Figure  3a. SbNRs are highly absorbing in the visible region, for they show a sharp peak with λ max at 250 nm, followed by a broad absorption band spanning from ∼300 to 800 nm. A similar broad and rather flat visible absorption band was observed in the past for SbNPs synthesized in solution phase using NaBH 4 as the reducing agent. 32 The corresponding Tauc plot ( Figure  S4b) of (αhν) 2 versus hν exhibits a linear dependence over 1.8−3.2 eV and follows the relation: (αhν) 2 = E g − hν, which is valid for direct gap semiconductors. Consequently, in SbNRs, the transitions from the VB to the conduction band CB are direct, as shown in the inset of Figure S4b. The intercept on the abscissa represents E g , the band gap of SbNRs, and it is found to be 1.65 eV. Pure SiNWs show a distinct broad peak in the visible region, which tapers off in the NIR region, and from the associated Tauc plot ( Figure S4a), the band gap is estimated to be 1.1 eV. The SbNRs@SiNW composite's absorption profile represents a combination of the individual absorption features of the two components. The HOMO position of SbNRs is obtained from cyclic voltammetry ( Figure S1 and Table S1, Supporting Information).
SbNRs were also found to be highly luminescent, and fluorescence spectra recorded at different excitation wavelengths of 350, 400, 450, and 500 nm are shown in Figure 3b. The quantum yield of SbNRs was found to be 0.41, and it was measured using a Rhodamine 6G dye solution as a reference ( Figure S5). More details are provided in the Supporting Information. Under λ ex = 350 nm, twin peaks at 403 and 427 nm followed by a shoulder at 454 nm were obtained; the intensity decreases and the peaks undergo a red shift at λ ex = 400 nm and the profiles are rather featureless under still longer excitation wavelengths. Since the band gap of SbNRs is 1.65 eV, and the observed emission peaks are observed at energies greater than the band gap, these peaks are attributed to excitation to higher energy levels in the CB followed by radiative transition to the VB maximum.
Excitation wavelength-dependent fluorescence is usually caused by the presence of polar groups in the fluorophore, like graphene oxide (GO) nanosheets, which typically have a high proportion of −COOH groups and the use of a polar solvent like water or alcohol. 33 It is very natural for polar −OH groups to be easily adsorbed on the surface of SbNRs during the final washing with ethanol/dilute acid/water media. Furthermore, the SbNRs, like GO, are also dispersed in a polar medium like isopropanol. The giant red-edge effect, i.e., the shift of emission maxima to longer wavelengths, in materials like GO in a polar solvent is ascribed to a slow solvent relaxation process, caused by the local chemical environment of the material, and considering the similarity of the role played by the surface groups, and the dispersion medium, the same reason can be assigned to this case as well.
The defect states in SiNWs play a crucial role in controlling the device performance. To understand more about defects, the fluorescence spectrum of SiNWs was measured under an excitation wavelength of 360 nm ( Figure 3c) and it showed a broad emission peak in the wavelength range of 400−600 nm. This peak lies over an energy range that is greater than the band gap of SiNWs, which is 1.1 eV. This higher energy emission is therefore due to the Si−O-based defects; these are surface defects formed by the oxidation of the SiNW surfaces. Contrary to planar Si, SiNWs are characterized by a significantly enhanced surface-to-volume ratio and this allows easy diffusion and adsorption of moisture and oxygen deep in between the wires, thus causing the oxidation of SiNW surfaces. The formation of Si−O−H, Si−O−O−Si, and Si− O−Si bonds occurs at the surfaces, and the energy levels of these defects lie at values that are positioned above the conduction band minimum, which explains the higher energy electronic transitions and hence the emission in the visible region. Similar observations have been made in the past for 1D-SiNWs prepared without a catalyst. 34 In another study on SiNWs prepared by laser ablation at high temperature, the emission in the visible region was attributed to the defect states produced by the amorphous silicon oxide layer formed over crystalline SiNWs. 35 More experimental evidence for the presence of defect states was obtained through the deconvolution of the core level Si2p XPS spectrum, shown in the Supporting Information (as Figure S6). It shows a distinct Si−O signal at 104 eV from the oxidized Si surface besides the spin-orbital components of Si2p 3/2 and Si2p 1/2 at 99.5 and 103 eV, respectively. The intensities are 8.14, 74.27, and 17.59%. The relatively good intensity of the Si−O peak indicates that defects are fairly prominent. These defects are passivated to some extent by coating TMSPA.
The composite shows a reduced fluorescence intensity compared to pristine SiNWs due to thermodynamically allowed excited electron injection from the CB of SbNRs (at −3.1 eV) to the CB (at −4.1 eV) of SiNWs. This is also depicted clearly in the energy band diagram shown in Scheme 1. Photoactive SbNRs act as cosensitizers, undergo electron− hole separation under irradiance, and provide additional charge carriers, thereby contributing to improving the photocurrent and the PCE of the solar cell. Time-resolved fluorescence decay curves of SbNRs and SiNWs and SbNRs@SiNWs were measured at λ em = 450 nm, and they are shown in Figure 3d. The decay profiles were fitted into a double exponential function, and the parameters are summarized in Table S2. The average lifetimes obtained for SiNWs, SbNRs, and SbNRs@ SiNWs are 3.2, 3.2, and 2.5 ns, respectively. The lifetime in pristine SbNRs or SiNWs represents the residence time in their respective CBs prior to radiative recombination.
The shorter lifetime of SbNRs@SiNWs is due to fast electron injection from SbNRs to SiNWs (if SiNWs are not present, then the lifetime is longer because band edge recombination is a slower process compared to electron transfer between the CBs). The fast transfer is due to the thermodynamically favorable positions of the CBs of SiNWs (−4.1 eV) and SbNRs (−3.44 eV). Under irradiance, the excited electrons are injected from the CB of SbNRs to the CB of SiNWs, and these electrons are transferred to the external circuit when used in a solar cell, thus accounting for the high efficiency of this cell. Therefore, the excited electron lifetime is shorter for SbNRs@SiNWs. Simultaneously, SbNRs show the p-type conduction mechanism, which was confirmed by a Mott−Schottky analysis. The 1/C 2 versus potential plot shows a linear behavior with a negative slope. The data is shown in Figure S7.
The VBs of SiNWs (−5.2 eV) and SbNRs (−5.09 eV) are well aligned for the hole transfer. Under irradiance, holes are also easily transported from SiNWs via SbNRs to the electrolyte. Thus, SbNRs increase the charge separation in the photoanode.
The dc electrical conductivities (σ) of SbNRs were measured under dark and light in the FTO/SbNRs/SS configurations (cartoon in Figure S8a), and the corresponding I−V plots are shown in Figure S8a. In this setup, a parafilm spacer of thickness "l" separated the two current collectors, and SbNRs were filled into a cavity of area "a" created at the center to prevent the shorting of the cell. White light illumination (100 mW cm −2 ) was done from the FTO side. While dark and light currents were not significantly different over the −1 to +0.5 V potential window, at ∼+0.5 V, a steep increase in current was registered only under illumination. Using linear fits over this region, the conductivities of SbNRs were estimated using Ohm's law, σ = (ΔI/ΔV) × l/a, and the values were 2 and 0.2 μS cm −1 , respectively. Charge separation under irradiance coupled with light-stimulated fast charge propagation possibly causes the sharp enhancement in current, thus imparting photoconductivity to SbNRs. This property is useful, for it can accelerate charge transport and transfer to SiNWs during solar cell operation. Conduction in dark in SbNRs is explained here. Sb is a metalloid comprising fused and ruffled, six-membered rings. The nearest and next-nearest neighbors form an irregular octahedral complex, with the 3-Sb atoms in each double layer relatively closer than the 3-Sb atoms in the adjacent one. The interlayer weak bonding allows electron transport between the layers, which is significantly amplified under irradiance. In the past, Sb-embedded carbon nanorods (CNRs) in a perovskite solar cell exhibited faster electron transport compared to sole CNRs. 36 Sb incorporation made the work function shallower and also offered additional light scattering in Sb-CNR, thus resulting in increased PCE. 36 To independently quantify the ability of SbNRs to serve as photosensitizers, J−V characteristics were measured for a film of SbNRs deposited over FTO (as the working electrode), and Pt rod as the CE, in dark and under 1 sun illumination in the presence of I − /I 3 − liquid electrolyte (cartoon in Figure S8b represents the cell configuration). The J−V plots in Figure S8b show that the photocurrent density increased by 19 times from 0.02 to ∼0.4 mA cm −2 and the photovoltage of the cell improved by 1.3 times from 0.30 to 0.38 V, ongoing from the performance in dark to that in light. This result proves that upon illumination semiconducting SbNRs undergo electron− hole separation, and the photoexcited electrons are transferred to FTO, and via the external circuit, they reach the Pt electrode, where I 3 − ions in the electrolyte reduce to I − ions at the Pt/electrolyte interface. Thereafter, I − ions diffuse to the SbNRs/FTO electrode, are oxidized, and regenerate the SbNRs, thus completing the circuit and the cell functions as a solar cell. All of these processes are triggered by illumination and occur spontaneously.
In the above equation, C is the capacitance, ε r is the dielectric constant (taken as 7.6 from an earlier report), 38 ε 0 is the permittivity of free space, E fb is the flat band potential, and k b is Boltzmann's constant. The slope is negative, implying a ptype conduction, and the magnitude is given by 2/(e × ε r × ε 0 × N d ). From the slope, the hole density (N d ) is estimated at 4.35 × 10 13 cm −3 . E fb corresponds to the intercept and is found  Figure 5 depict the cell configurations employed in each measurement. CV plots were recorded in a 3-electrode cell with two Pt rods as the WE and CE and a Ag/AgCl/KCl as the RE immersed in the said electrolyte (Figure 5a). The CV plots show a broad reduction peak in the cathodic branch corresponding to the following reaction: 1/2I 3 − + e − = 3/ 2I − , which were observed at 0.54, 0.24, and 0.46 V versus SHE, respectively. Interestingly, the Cu 2 O-gel shows reduction of I 3 − ions at a higher positive potential relative to the pristine gel, which proves that the I 3 − species can be more spontaneously reduced to I − in the presence of Cu 2 O. Furthermore, the peak current densities are comparable for Cu 2 O-gel and the liquid electrolytes and higher than that of the gel, thus indicating that the propensity of Cu 2 O-gel to electrocatalyze I 3 − reduction is quite high. The standard reduction potential for I 3 − to I − reduction is 0.35 V versus SHE. 39 The oxidation peak is only observed for the Cu 2 O-gel electrolyte at 0.50 V versus NHE.
Iodine being a halogen atom is very reluctant to undergo oxidation, and it generally has a strong tendency to readily capture one electron to convert to I − , i.e., the overpotential for the reverse reaction, iodide conversion to iodine is very large. So, for this reason in most of the electrolytes, only the reduction peak is observed. In the case of the Cu 2 O-gel, it is apparent that Cu 2 O possibly alters the interfacial composition to make it more conducive for iodide oxidation.
LSV studies were performed with NiO as the WE, Pt as the CE, and Ag/AgCl/KCl as the RE in liquid, gel, and Cu 2 O-gel electrolytes (Figure 5b). The onset of reduction occurs at 0.54, 0.48, and 0.37 V versus SHE for the liquid, Cu 2 O-gel, and gel electrolytes, respectively, which matches the outcome of CV analysis. The overpotential of reduction for liquid, Cu 2 O-gel, and gel electrolytes is 0.19, 0.13, and 0.02 V, respectively. NiO provides a larger active area for interaction with the electrolyte due to its mesoporous structure. Also, it acts as a highly electrocatalytic material allowing fast electron injection into the electrolyte. It has also been widely used in dye-sensitized solar cells as a photocathode. 40 Finally, with NiO, a greatly enhanced PCE is obtained, thus justifying its use as a CE.
Nyquist plots for symmetrical cell configurations with two Pt electrodes immersed in liquid, gel, and Cu 2 O-gel are shown in Figure 5c. The data were fitted into an [R(RC)W] circuit. The charge-transfer resistances (R ct ) for the cells are ∼32, ∼118, and ∼47 Ω cm 2 , respectively. R ct is the highest for the gel and lowest for the liquid electrolyte, and the Cu 2 O-gel gives a value close to that of the liquid. In the gel, the polymer, PEO, retards charge transport, which possibly affects both R ct and also makes the I 3 − reduction potential more negative, as seen earlier. However, when Cu 2 O NCs are dispersed in the gel, due to their intrinsic hole-conducting capability, they offset the effect of PEO, enhance charge transport, and also facilitate charge transfer at the Pt/electrolyte interface. The oxidation peak obtained for Cu 2 O versus Ag/AgCl is +1.1 V. Hence, the HOMO level is at −5.79 eV. The HOMO level of Cu 2 O is perfectly aligned with the HOMO level of NiO at −5.3 eV for electron conduction to take place. The electron from the VB of NiO gets quickly captured by the VB of p-type Cu 2 O, which helps in lowering the charge-transfer resistance at the counter electrode/electrolyte interface. This is the reason why the R ct drops drastically upon Cu 2 O addition to the gel electrolyte.
Furthermore, their high degree of crystallinity also imparts additional mechanical strength to the gel, thus enabling the fabrication of quasi-solid-state solar cells. With NiO//NiO symmetric cells, the same performance was repeated albeit slightly higher values registered for the charge-transfer resistances. From Figure 5d, it can be gauged that the R ct magnitudes are higher compared to Pt//Pt cells. Compared to Pt, the sheet resistance of p-type NiO is significantly larger than that of Pt, and this impacts the R ct value. The fitted parameters for NiO//NiO and Pt//Pt cells with different electrolytes are furnished in Table S3.
Properties of NiO. NiO/FTO was deployed as the CE in the SiNW-based solar cells. The NiO film was deposited over FTO in chronoamperometric mode, with Ag/AgCl/KCl as the RE and a Pt rod as the CE, by applying a constant dc potential for 10 min (cartoon in Figure S9a represents the cell used). The current versus time transient is shown in Figure S9a The initial spike in current is due to the reduction of the nitrate ions at the working electrode, which leads to the formation of OH − . The OH − ions react with Ni 2+ cations to produce Ni(OH) 2 . The formation of insoluble Ni(OH) 2 at the working electrode leads to a lowering in current. After some time, the current saturates and then increases as the ion diffusion process predominates.
Current versus t −1/2 plot is provided as an inset. Using the Cottrell equation and from the linear fit shown in Figure S9a, the diffusion coefficient (D) for the migrating species was determined to be 7.99 × 10 −12 cm 2 s −1 Here F is 96 500 C/mol, A is the active area, C is the concentration of the analyte, and n is the number of electrons involved in the redox process.
The UV−visible absorption spectrum of NiO ( Figure S9b) shows a strong λ max at 312 nm, and the optical band gap calculated from the equation E g = 1240/λ for NiO is 3.6 eV, and it is an indirect band-gap semiconductor. 42 The XRD pattern of NiO ( Figure S9c Figure S9d) shows that it is a p-type semiconductor with a hole density of 9.40 × 10 13 /cm 3 calculated from eq 5, where ε r of NiO was taken as 12 from a previous report. 43 FE-SEM images ( Figure S10a−c) of NiO clearly bring out the mesoporous structure, as the oxide appears to be composed of irregularly shaped particles separated by pores to 10−30 nm in dimensions. TEM images show that the particles are composed of cubic or cuboidal shapes, with size in the range of 30−90 nm ( Figure S10d). An interfringe distance of 0.20 nm is observed in a high-resolution image ( Figure  S10e), and it aligns with the (220) reflection of the fcc phase of NiO. The SAED pattern ( Figure S10f) shows scattered spots, which are indexed to the (220), (311), and the (222) planes of the same structure. These results are in good agreement with the XRD data. The -OH-functionalized SiNWs are highly vulnerable in liquid electrolyte. To prevent the degradation of such -OH functional groups via electrolyte corrosion and photocorrosion, it is very important to functionalize the -OH groups on the surface. N- propyl]aniline or TMSPA coating on asfabricated SiNWs leads to the formation of a covalently linked self-assembled monolayer over the SiNW surface through the O−Si bonds. The mechanism is shown in Scheme S2, and because the molecule is only a few nanometers in size, this monolayer is a few nanometers thick and is invisible to the naked eye.
XPS analysis of SbNRs@TMSPA@SiNWs was performed to clearly understand the coordination between TMSPA and SiNWs ( Figure S6, Supporting Information). The full survey spectrum shows the presence of Si 2p, Sb 4s, N 1s, O 1s, and Sb 3d at 99, 148, 401, 530, and 539 eV, respectively. The core level of Sb 3d upon deconvolution shows the presence of Sb 3d 5/2 , Sb 3d 3/2 , and O 1s at 526, 535, and 529 eV. The obtained values are very similar to the previously reported values. 44 The relative intensities of Sb 3d 5/2 and Sb 3d 3/2 are 33.27 and 18.09%, respectively. The core level spectrum of N1s show one asymmetric peak, and upon deconvolution, the peaks are assigned to the C−N and N−H bonds (arising specifically from the TMSPA) at 399 and 401 eV, respectively. The relative intensities of C−N and N−H bonds are 86.2 and 13.8%, respectively. The presence of N1s peaks in TMSPA@ SbNRs@SiNWs even after the cleaning step confirms the bonding between TMSPA and SiNWs.
The J−V characteristics of TMSPA@SiNWs/Cu 2 O-gel/NiO and SiNWs/Cu 2 O-gel/NiO solar cells, essentially with and without the TMSPA monolayer, are compared in Figure 6a. The process improves solar cell stability and operational lifetime, without compromising the PCE for they are 2.7 and 2.8%, thus unambiguously illustrating that it is a powerful yet simple technique to enhance cell stability, an issue of great concern, particularly with regard to commercial prospects. The solar cell parameters are provided in Table 1, and the five cell average data with standard deviation is given in Table S4. The obtained standard deviation value is close to the value obtained by Ahn et al. 45 for their perovskite solar cells. It is observed that the variation in the values ongoing from one cell to another is not much; this is probably because here a liquid or a gel electrolyte is used as the HTL. With such an HTL, deep penetration into the photoanode and CE is ensured because they are porous electrodes. This results in superior interfacial properties compared to solid-state cells (like organic solar cells), and multiple cells can therefore show performance parameters that are close in values.  SbNRs showed V OC = 771 mV, J SC = 12.9 mA cm −2 , FF = 0.72, and PCE = 7.2%. With the liquid electrolyte and with SbNRs, not only a high PCE is obtained, but a high FF, in excess of 70% is achieved, which is a remarkable achievement. In the past for SiNW-based LJSCs, such a high FF has rarely been observed; 1,3,6 FFs typically of about 40−55% have been obtained in the best SiNW cells. J SC increases by ∼12% for SbNR-decorated solar cells. This increase in current is due to the p-type hole transport property, which lowers the electron− hole recombination at the photoanode/electrolyte interface. Thus, SbNRs assist in improving charge separation, and this is reflected in the FF increment, which increases quite significantly by ∼47%. A very high J SC with a low FF generally does not represent a good solar cell. Since here a good balance between both J SC and FF is achieved with the addition of SbNRs, this cell has strong potential for practical applications.

Effect of Cu 2 O NCs in the
SbNRs are semiconducting, and as shown earlier, they are highly fluorescent with strong emission in the visible region and are therefore capable of undergoing charge separation under irradiance. The excited charge carriers are then rapidly transferred to the CB of SiNWs, permitted thermodynamically by the energy level offsets of SiNWs and SbNRs. SiNWs then transmit these electrons to the external circuit. Simultaneously, the holes produced in SiNWs transfer to SbNRs and then to I − in the electrolyte, maximizing charge separation and suppressing electron−hole recombination, thereby maximizing the PCE, which is 7.2%. This leads to efficient light-generated carrier separation and a smooth single pathway for hole transportation via favorably aligned energy levels. These factors also reduce the recombination at the photoanode/electrolyte interface and improve solar cell efficiency compared to the pristine SiNW photoanode-based solar cell, wherein recombination can occur very easily. The liquid nature of the electrolyte is also beneficial, for PCE was only 2.2% with gel, and it is 7.2% with the liquid, having the same photoanode of SbNRs@TMPSA@SiNWs and a CE of NiO/FTO. While the gel enables the fabrication of a quasi-solid-state cell with ease, which is also relatively easier to handle, but the liquid offers better permeation properties, it is able to easily infiltrate the cross sections of the photoanode (be it SiNWs or SbNRs@ SiNWs) and the mesoporous NiO CE, thus maximizing the interaction between the VBs of the photoanode and the I − ions. The deep penetration of the liquid electrolyte compared to that of gel allows for more charge separation improving the PCE.
Despite the high PCE, the liquid electrolyte poses several practical issues, which restricts its commercial applicability. Under illuminated conditions, the solvent from the liquid electrolyte can evaporate or leak from the edges of the device, thus lowering the PCE. In contrast, the gel electrolyte can be visualized as a liquid electrolyte immobilized in a solid phase (polymer). Therefore, it offers almost liquid-like conductivity and the polymer chains trap the solvent molecules and prevent their evaporation, thus imparting long-term device stability. The stability of the liquid electrolyte-based cell is recorded (cell efficiency drops from 7.17 to 1.76%) and is shown in Figure S11 in the Supporting Information. The solar cell parameters are compared in Table 1, and the five cell average data with standard deviation is shown in Table S4. A comparison with literature values of cells with SiNW-based photoanodes is provided in Table S6, and it shows that our value in terms of attaining a good trade-off between stability and efficiency is quite good.
Effect of p-Type NiO. The effect of NiO as the CE is discerned by comparing the J−V characteristics of two cells with the same photoanode and the electrolyte but different CEs (NiO/FTO and FTO) (Figure 6e). The TMSPA@ SiNWs/liquid/NiO/FTO cell showed V OC = 791 mV, J SC = 11.5 mA cm −2 , FF = 0.49, and PCE = 4.4% and the TMSPA@ SiNWs/liquid/FTO cell showed V OC = 890 mV, J SC = 0.4 mA cm −2 , FF = 0.08, and PCE = 0.03%. The PCE increases by 147-times on replacing FTO with NiO/FTO. The reason for this huge jump in PCE is this: FTO is just a current collector and does not offer any electrocatalytic properties, whereas NiO is highly electrocatalytic and also furnishes a high reaction surface area by the virtue of its mesoporous morphology (as seen earlier in the FE-SEM images). Consequently, during solar cell operation, a very large number of I 3 − species are adsorbed at the NiO/electrolyte interface and also undergo reduction very easily, for the VB of NiO is more negative relative to the work function of FTO. During the cell operation, the electrons from the photoanode reach the CE, and from FTO, they cascade to the VB of NiO, and from the VB, they are quickly picked up by the I 3 − ions that are in direct contact with the NiO surface. FTO is a planar current collector and does not offer this benefit; as a consequence, the electron injection efficiency is poor at the FTO/electrolyte interface, and the PCE is very low. The optical transmittance of FTO is higher than that of NiO, as shown in Figure S12. The higher optical transmittance in the FTO electrode results in better light penetration, thus amounting to a better charge buildup in the SiNW photoanode. As a consequence, the TMSPA@ SiNWs/liquid/FTO cell shows greater photovoltage. In comparison, the NiO CE shows an optimal balance between optical transmittance and electrocatalytic property, and this advantage trumps the above aspect of FTO, thus resulting in an overall higher PCE with NiO.
NiO/FTO absorbs some part of the incoming solar radiation. These losses in incoming light can be further adjusted by optimizing the thickness of the NiO layer, which will be attempted in future. A thin film of NiO can provide high light transmittance in visible and NIR regions. This can be achieved by lowering the electrodeposition time of the NiO film. Hammad et al. prepared NiO on the glass substrates/Si wafer by DC-sputtering method. 46 They showed that the optical band gap of NiO films decreases as the film thickness increases; the thin film showed higher and broader UV− visible−NIR optical transmittance compared to the thick films. Chen et al. also reported the preparation of NiO films by DC magnetron sputtering. 47 They showed that the films prepared at 100 W for 10 min showed the best optical transmittance (∼80%) over the visible−NIR region.
The solar cell parameters are compared in Table 1, and the five cell average data with standard deviation is shown in Table  S4. Additionally, a J−V measurement was also performed with a Pt/FTO CE. SbNRs@TMSPA@SiNWs/Cu 2 O-gel/Pt@ FTO cell and the J−V diagram are shown in Figure S13 Figure S14 (Supporting Information), and the PV parameters are summarized in Table  S7. It has been observed that the device efficiency increases from 3.05 to 4.7% upon increasing the etching time from 5 to 15 min. This could be due to the increase in the antireflective properties of the SiNWs upon an increase in wire length due to higher etching time. However, a further increase in etching time leads to the formation of ultralong nanowires, which get heavily bundled due to their long lengths. The bundling of nanowires causes poor electrolyte as well as light penetration into the electrode. This results in less charge separation and lower device efficiency and at 20 min etching time, the efficiency drops to 3.4%. A similar trend has been reported previously by many groups. 12−14 The optimal average length of 3−7 μm is therefore achieved at an etching time of 15 min, which is the case here.
Comparison of EQEs. The EQE spectra of SbNRs@ TMSPA@SiNWs/Cu 2 O-gel/NiO@FTO and TMSPA@ SiNWs/gel/NiO@FTO cells are now shown in Figure S15 (Supporting Information). The comparison shows that with SbNRs and Cu 2 O, a larger EQE is registered over the 400− 1040 nm wavelength region. The EQE maxima with and without SbNRs and Cu 2 O are 38.7 and 9.7% at 880 nm, respectively.
Stability Analysis. SiNW-based photoelectrochemical cells showed serious stability issues in the past. Lewis et al., in the year 2010, reported a SiNW-based solar cell where they used a viologen-based electrolyte, which showed a 3% efficiency but poor stability. 48 In the past, the ferrocene/ferrocenium redox couple electrolyte dissolved in acetonitrile or methanol or water has been explored but it also suffered from poor stability. 49−53 Several research groups reported the poor stability of SiNW-based photoelectrochemical cells in the aqueous HBr-Br 2 electrolyte. 1,3 The poor stability of the aqueous Br 2 /Br − electrolyte-based device is attributed to the corrosive nature of HBr mineral acid and liquid bromine. SiNWs tend to rapidly dissolve in this corrosive medium, thus restricting the use of the solar cell for any practical application. Another point of concern with an aqueous electrolyte-based cell is that the SiNW photoanode gets partially oxidized to SiO 2 , which is highly resistive and causes the efficiency of the device to decline with time. Use of nonaqueous organic electrolytes based on solvents like propylene carbonate can solve the problem. In 2010, Shen et al. reported a photoelectrochemical cell where they used LiI and I 2 dissolved in EMISCN and PMII ionic liquid electrolytes and the cell showed excellent stability under 1 sun 2 . The ionic liquids EMISCN and PMII show high thermal and chemical stabilities and they are also nonvolatile, which ensures that the composition of the electrolyte does not change with time. Aqueous electrolytes have a tendency to evaporate through the crevices causing the PCE to decline with time. In this work, we used a poly(ethylene oxide) or PEO-based gel electrolyte with LiI and I 2 dissolved in EMISCN, PMII, and propylene carbonate with Cu 2 O NCs dispersed therein. The electrolyte synthesis is accomplished at low temperatures and it is a very stable electrolyte.
After 500 h of continuous illumination, the PCE of the SbNRs@TMSPA@SiNWs/Cu 2 O-gel/NiO@FTO cell dropped by ∼24.4% and the efficiency of the TMSPA@SiNWs/ Cu 2 O-gel/NiO@FTO device dropped by ∼70.1%. Hence, the SbNRs not only improve the device efficiency but also passivate the SiNW surface by capturing the holes from SiNWs and protecting it from electrolyte corrosion. The stability data of the SbNRs@TMSPA@SiNWs/Cu 2 O-gel/ NiO@FTO cell is presented in Table S8 of the Supporting Information.
The PCE of the TMSPA@SiNWs/Cu 2 O-gel/NiO@FTO device dropped by ∼70.1%, and the PCE of the SiNWs/Cu 2 Ogel/NiO@FTO device dropped by ∼96.8%. The higher drop in PCE of the device without TMSPA shows a greater drop in efficiency after 500 h. The lower drop in PCE in the TMSPA@ SiNWs/Cu 2 O-gel/NiO@FTO cell is due to the self-assembled monolayer formation of TMSPA, which protects the SiNWs from photo-oxidation and electrolyte corrosion. The data is presented in Table S9 of the Supporting Information.
Charge-Transfer/Transport Phenomena in Solar Cells. Nyquist plots of SbNRs@TMSPA@SiNWs/liquid/ NiO under light and dark are shown in Figure S16a and are fitted into R(QR)(QR) and R(RQ)W circuits. Under illuminated conditions, the bulk resistance (R b ) = 128 Ω cm 2 , R ct at the CE/electrolyte interface = 185 Ω cm 2 , and recombination resistance (R rec ) at the photoanode/electrolyte interface = 12.4 kΩ cm 2 . In dark, the R b is unaltered, but R ct = 5.4 kΩ cm 2 . R ct increased by 29 times in dark compared to that in light. This increase in resistance is due to poor electron transport to counter electrode from photoanode under dark conditions. Nyquist plots of SbNRs@TMSPA@SiNWs/Cu 2 Ogel/NiO@FTO under light and dark are shown in Figure  S16b, and the plots are fitted into an R(QR)(QR) circuit. Under illuminated conditions, the R b = 59 Ω cm 2 , R ct = 884 Ω cm 2 , and R rec = 3650 Ω cm 2 and in dark R b = 56 Ω cm 2 , R ct = 1125 Ω cm 2 , and R rec = 5855 Ω cm 2 . The R ct value increased 1.3 times, and R rec value increased 1.6 times in dark compared to those in light. Under irradiance, the excited charged carriers recombine more compared to dark and as a result the cell showed higher recombination and charge-transfer resistance compared to light. The fitted parameters of all of the cells are summarized in Table S10. The Bode plots of SbNRs@ TMSPA@SiNWs/liquid/NiO and SbNRs@TMSPA@SiNWs/ Cu 2 O-gel/NiO cells under dark and light are shown in Figure  S16c. The recombination lifetime (τ) of the Cu 2 O-gel-based cell under dark and illuminated conditions is 11.3 and 0.6 ms, respectively, and the recombination lifetime with the liquid under illuminated condition is 24.1 ms.

■ CONCLUSIONS
A quasi-solid-state solar cell with the following architecture, SbNRs@TMSPA@SiNWs/Cu 2 O-gel/NiO solar cell, was fabricated, and it delivered a PCE of 4.7%, superior by 177% compared to the cell without SbNRs or Cu 2 O NCs. Upon irradiance, SbNRs undergo electron−hole separation and inject excited electrons to SiNWs, while simultaneously accepting holes from them as well, thus minimizing recombination at the photoanode/electrolyte interface and improving photocurrents. Cu 2 O NCs, which are homogeneously dispersed in the gel matrix, also have a favorably poised Fermi level, allowing the unhindered extraction and transmission of holes from SbNRs or SiNWs to the I − ions in the electrolyte. Electrocatalytic and mesoporous NiO as the CE affords substantial interaction with the I 3 − ions and maximizes their reduction. The combined effect of these factors manifests the high PCE, and the TMSPA overlayer efficiently prevents the photocorrosion of SiNWs, enabling the cell to sustain 500 h of extended and intermittent illumination, with a 24.4% decline in its PCE. This study demonstrates the potential of using low-cost photoactive and/or semiconducting materials with favorably aligned energy levels for developing efficient solar cells, and these materials can easily conform to other solar cells as well.